Interstellar comet 3I/ATLAS formed in a deep-freeze environment near or below 30 kelvins, according to research led by Northumbria University scientists using the William Herschel Telescope. By analyzing the object’s plasma tail with the WEAVE instrument, researchers detected a high concentration of molecular nitrogen alongside four other ion species, offering an unprecedented look at planetary building blocks from another star system.
Detecting Five Ion Species in an Interstellar Tail
Using the WEAVE instrument attached to the 4.2-meter William Herschel Telescope, scientists spotted five different ion species—including ionized molecular nitrogen—at the same time within the plasma tail of interstellar comet 3I/ATLAS. Discovered on July 1, 2025, the comet is only the third confirmed interstellar object to enter our solar system, following 1I/‘Oumuamua and 2I/Borisov.
Researchers observed the tail on Nov. 30 and Dec. 2, 2025, after the object passed perihelion. According to lead researcher Lea Ferellec of Northumbria University, the team used the Large Integral Field Unit mode of WEAVE to collect spectral information across a two-dimensional area. This setup allowed scientists to separate the faint ion tail spatially from the bright dust and neutral gas surrounding the comet.
The observations revealed N2+, CO+, CO2+, H2O+, and CH+ at the same time. Individual ion species had not previously been spectroscopically characterized this way in an interstellar visitor, according to the research team.
A Cosmic Thermometer Points to a Frigid Birthplace
Because the nitrogen signature is unusually strong compared to carbon monoxide, scientists believe the body originated in environments chilled to around 30 kelvins or less (roughly minus 243 degrees Celsius). Molecular nitrogen is exceptionally volatile and difficult to retain in cometary ice, making the relative abundance of nitrogen and carbon monoxide a reliable thermometer for original formation environments.
“This object gives us a rare chance to study material that formed somewhere completely different from our own solar system,” Lea Ferellec said. “Finding that it’s so rich in nitrogen tells us it likely formed in extremely cold conditions, far from its home star.”
The researchers measured an N2+/CO+ abundance ratio of about 2.3% in the plasma tail. This nitrogen enrichment places 3I/ATLAS among a small group of unusually nitrogen-rich comets, indicating that its ices assembled in the remote outer portions of a distant protoplanetary disk.
Did you know? 3I/ATLAS is only the third confirmed interstellar object ever detected passing through our solar system, following 1I/‘Oumuamua and 2I/Borisov. Unlike ‘Oumuamua, 3I/ATLAS displayed classic cometary activity, venting gases and dust as it neared the Sun.
Plasma Tail Dynamics and Chemical Stability
Providing a uniquely thorough perspective on the plasma chemistry of an interstellar comet, readings taken along the tail demonstrated that most ion ratios stayed consistent across the examined distances, except for a minor drop in CH+. When the team divided observations from Dec. 2 into regions closer to and farther from the nucleus, N2+, CO+, CO2+, and H2O+ showed no significant changes.
Isaac Newton Group Director Rubén Sánchez-Janssen noted that the rapid deployment of these observations highlighted the value of DDT. “This discovery is a perfect example of the value of DDT, which is specifically designed to enable observations of exceptional and urgent scientific importance,” Sánchez-Janssen stated.
The stability of the primary ion ratios confirms that the measured nitrogen-to-carbon-monoxide comparison provides a steady snapshot of the comet’s original parent material, despite the complex plasma environment of the tail.
Frequently Asked Questions
What makes comet 3I/ATLAS unique?
It is only the third confirmed interstellar object to enter our solar system and the first whose plasma tail has been spectroscopically resolved to simultaneously show five distinct ion species.
How do astronomers know the comet formed at 30 kelvins or below?
The high abundance of volatile molecular nitrogen relative to carbon monoxide in the plasma tail acts as a chemical thermometer, indicating exceptionally cold formation temperatures in the outer reaches of a foreign planetary system.
What instrument was used to make this discovery?
Researchers used the WHT Enhanced Area Velocity Explorer (WEAVE) instrument and its Large Integral Field Unit mounted on the 4.2-meter William Herschel Telescope.
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